Last's Anatomy: Regional and Applied

Part five. Vessels and nerves of the gut

Blood supply of the foregut

Coeliac trunk

This is the artery of the foregut, and its three branches—the left gastric, splenic and common hepatic arteries—supply not only the gut from the lower part of the oesophagus down to the opening of the bile duct into the duodenum, but also the foregut derivatives (the liver and pancreas) and the spleen. It arises from the front of the abdominal aorta between the crura of the diaphragm a little below the median arcuate ligament, at the level of the body of T12 vertebra. It is usually a short wide trunk, flanked by the coeliac group of preaortic lymph nodes. The coeliac ganglia lie one on each side and they send to the artery sympathetic nerves which are carried along all its branches.

At the upper border of the pancreas the trunk divides into its three branches behind the peritoneum of the posterior wall of the lesser sac (Fig. 5.22).

The left gastric artery runs upwards across the left crus towards the oesophageal opening in the diaphragm. It gives off oesphageal branches and turns anteroinferiorly, raising a small fold of peritoneum, the left gastropancreatic fold. It then runs to the right in the lesser omentum along the lesser curvature and supplies the stomach.

The splenic artery passes to the left. It is characteristically very tortuous; the crests of its waves appear above the pancreas, and the troughs lie hidden behind its upper border. It runs across the left crus and left psoas to the hilum of the left kidney, where it turns forward in the splenorenal ligament to the hilum of the spleen. Apart from the spleen it is the main supply to the pancreas. Before breaking up into its terminal splenic branches it gives off about 6 short gastric arteries which run in the gastrosplenic ligament, and the left gastroepiploic artery which runs to the right in the greater omentum, a little distance away from the greater curvature, from where it gives branches to the stomach and the omentum. From the middle part of its course the splenic artery may give off a posterior gastric artery to the stomach, which raises a fold of parietal peritoneum, the gastrosplenic ligament, as it arches forwards towards the stomach.

The common hepatic artery passes over the upper border of the pancreas, downwards and to the right behind the peritoneum of the posterior abdominal wall as far as the first part of the duodenum. It then turns forward, raising a small fold of peritoneum, the right gastropancreatic fold, and curves upwards between the two layers of the lesser omentum as the hepatic artery. Here it meets the bile duct and lies on its left side, both in front of the portal vein surrounded by the peritoneum at the free edge of the lesser omentum. On reaching the porta hepatis, the hepatic artery divides into right and left branches to supply the right and left halves of the liver. These branches and the associated aberrant or accessory hepatic arteries are described on page 262.

The common hepatic usually gives off the right gastric and gastroduodenal arteries.

The right gastric artery leaves the common hepatic as it turns forwards into the lesser omentum. It runs to the left along the lesser curvature and anastomoses with the left gastric artery.

The gastroduodenal artery passes down behind the first part of the duodenum, where it may be eroded by a duodenal ulcer. At the lower border of the duodenum it divides into two. The right gastroepiploic artery passes forward between the first part of the duodenum and the pancreas, and turns to the left between the two leaves of the greater omentum. It runs close to the greater curvature of the stomach and anastomoses with the left gastroepiploic artery.

The other branch of the gastroduodenal artery is the superior pancreaticoduodenal artery. It divides into a smaller anterior and a larger posterior branch, which may arise directly from the gastroduodenal artery; they anastomose with similar branches of the inferior pancreaticoduodenal branch of the superior mesenteric artery. The pancreaticoduodenal arteries supply the duodenum, head of the pancreas and bile duct. The entrance of the bile duct marks the junction of foregut and midgut, and is the meeting place of the arterial distributions of their respective arteries, coeliac and superior mesenteric.

One or two small supraduodenal arteries may arise from the common hepatic artery or its branches.

Venous drainage of the foregut

Right and left gastric, right and left gastroepiploic and the short gastric veins run with the corresponding arteries. All this blood reaches the liver via the portal vein (see p. 266) and, with the arterial blood of the hepatic artery, passes through the liver to be carried via the hepatic veins to the inferior vena cava.

The lower third of the oesophagus in the posterior mediastinum drains downwards by oesophageal veins, through the oesophageal opening in the diaphragm, to the left gastric vein. The oesophagus above this level drains into the azygos system of veins.

The left gastric vein runs to the left along the lesser curvature up to the oesophagus, then passes medially behind the peritoneum of the posterior wall of the lesser sac to join the portal vein at the upper border of the first part of the duodenum. The right gastric vein runs along the lesser curvature to the pylorus and empties into the portal vein. It receives the prepyloric vein which ascends in front of the pylorus.

The short gastric and left gastroepiploic veins run with the arteries through the gastrosplenic ligament and greater omentum to the hilum of the spleen, where they empty into the splenic vein.

The splenic vein begins in the hilum of the spleen by confluence of half a dozen tributaries from that organ. Having received the short gastric and left gastroepiploic veins, it passes to the right with the tail of the pancreas, below the splenic artery, in the splenorenal ligament and continues to the right, posterior to the body of the pancreas (Fig. 5.26), which it grooves. In its course it lies on the hilum of the left kidney, the left psoas muscle and left sympathetic trunk, the left crus of the diaphragm, the aorta and superior mesenteric artery and the inferior vena cava. It lies in front of the left renal vein along the upper border of that vessel. In front of the inferior vena cava it joins the superior mesenteric vein at a right angle to form the portal vein. It receives many tributaries from the tail, body, neck and head of the pancreas. As it lies in front of the left crus of the diaphragm it receives the inferior mesenteric vein from the hindgut.

The right gastroepiploic vein runs to the right in the greater omentum and descends over the front of the pancreas to join the superior mesenteric vein at the lower border of the neck of the pancreas.

The superior pancreaticoduodenal vein ascends behind the head of the pancreas to join the portal vein at the upper border of the pancreas.

Blood supply of the midgut

Superior mesenteric artery

This is the artery of the midgut and supplies the gut from the entrance of the bile duct to a level just short of the splenic flexure of the colon. The artery arises from the front of the aorta a centimetre below the coeliac trunk, at the level of the lower border of L1 vertebra. It is directed steeply downwards behind the splenic vein and the body of the pancreas, with the superior mesenteric vein on its right side. It lies anterior to the left renal vein, the uncinate process of the pancreas and the third part of the duodenum, in that order from above downwards (Fig. 5.26). With its vein it enters the upper end of the mesentery of the small intestine and passes down to the right along the root of the mesentery (Fig. 5.23). Pressure of the superior mesenteric artery on the left renal vein may produce left-sided varicocele, and pressure on the duodenum may give symptoms of chronic duodenal ileus, particularly when the angle between the artery and the aorta is smaller than usual.

The inferior pancreaticoduodenal artery is its first branch, arising from the posterior surface; it may come off the first jejunal branch. It divides into anterior and posterior branches which run in the curve between the duodenum and the head of the pancreas, supply both, and anastomose with the terminal branches of the superior pancreaticoduodenal artery.

The jejunal and ileal branches arise from the left of the main trunk and pass down between the two layers of the mesentery. The pattern of anastomosing arcades from which vessels enter the gut wall is described with the jejunum and ileum (see p. 254).

The ileocolic artery (Fig. 5.23) arises from the right side of the superior mesenteric trunk low down in the base of the mesentery. It descends to the right iliac fossa and divides into superior and inferior branches. The superior branch runs up along the left side of the ascending colon to anastomose with the right colic artery. The inferior branch runs to the ileocolic junction, and gives off anterior and posterior caecal arteries, an appendicular artery, and an ileal branch which ascends to the left on the ileum to anastomose with the terminal branch of the superior mesenteric artery.

The right colic artery (Fig. 5.23) arises from the right side of the superior mesenteric artery, or in common with the ileocolic artery. It runs to the right across the right psoas muscle, gonadal vessels, ureter and genitofemoral nerve, and quadratus lumborum, just behind the peritoneal floor of the right infracolic compartment. It divides near the left side of the ascending colon into two branches. The descending branch runs down to anastomose with the superior branch of the ileocolic artery. The ascending branch runs up across the inferior pole of the right kidney to the hepatic flexure where it anastomoses with a branch of the middle colic artery.

The middle colic artery (Fig. 5.23) arises from the right side of the superior mesenteric artery, as the artery emerges at the lower border of the neck of the pancreas, and descends between the two leaves of the transverse mesocolon. It lies to the right of the midline and at the intestinal border of the transverse mesocolon it divides into right and left branches which run along the margin of the transverse colon. The right branch anastomoses with the ascending branch of the right colic artery. The left branch supplies the transverse colon almost to the splenic flexure (the distal part of the midgut) where it anastomoses with a branch of the left colic artery. As the middle colic lies to the right of the midline it leaves a large avascular window to its left in the transverse mesocolon (Fig. 5.23). This window is the site of election for surgical access to the lesser sac and the posterior wall of the stomach.

Venous drainage of the midgut

Each branch of the superior mesenteric artery is accompanied by a vein. All these veins flow into the superior mesenteric vein, a large trunk which lies to the right of the artery. It crosses the third part of the duodenum and the uncinate process of the pancreas. Behind the neck of the pancreas it is joined by the splenic vein to form the portal vein. This continues upwards behind the first part of the duodenum. The superior mesenteric and portal veins represent a single continuing venous trunk, named portal vein above, and superior mesenteric vein below, the level of entry of the splenic vein (Fig. 5.26).

Blood supply of the hindgut

Inferior mesenteric artery

This is the artery of the hindgut; its area of supply extends as far as the upper third of the anal canal, i.e. to the level of the pectinate (dentate) line (see p. 315). The inferior mesenteric artery arises from the front of the aorta behind the inferior border of the third part of the duodenum, opposite L3 vertebra, at the level of the umbilicus, 3 or 4cm above the aortic bifurcation. It is smaller than the superior mesenteric artery. It runs obliquely down to the pelvic brim, immediately beneath the peritoneal floor of the left infracolic compartment. It gives off the left colic and sigmoid arteries. It crosses the pelvic brim at the bifurcation of the left common iliac vessels over the sacroiliac joint, at which point it converges towards the ureter, with the inferior mesenteric vein lying between them, at the apex of the Λ-shaped attachment of the sigmoid mesocolon. Its branches cross to the left in front of the ureter and the other structures in the posterior abdominal wall of the left infracolic compartment. Beyond the pelvic brim it continues in the root of the sigmoid mesocolon as the superior rectal artery (see p. 294).

The left colic artery leaves the trunk and passes upwards and to the left behind the peritoneum. After a short course it divides into an ascending and a descending branch. The ascending branch continues laterally and upwards, crossing the left psoas muscle, gonadal vessels, ureter and genitofemoral nerve, and quadratus lumborum. It is crossed anteriorly by the inferior mesenteric vein. The descending branch passes laterally and downwards. The branches of these two arteries anastomose with each other as well as (above) with the left branch of the middle colic artery and (below) with the highest sigmoid artery, thus contributing to an anastomotic channel along the inner margin of the colon (Fig. 5.24 and p. 258).

The sigmoid arteries are two to four branches which pass between the layers of the sigmoid mesocolon, in which they form anastomosing loops. The last sigmoid branch anastomoses with the first branch of the superior rectal artery.

Venous drainage of the hindgut

The superior rectal vein runs up in the root of the sigmoid mesocolon, on the left of the superior rectal artery, to the pelvic brim, above which it is named the inferior mesenteric vein. This receives tributaries identical with the branches of the inferior mesenteric artery. The vein itself runs vertically upwards well to the left of the artery, beneath the peritoneal floor of the left infracolic compartment. It lies on the left psoas muscle, in front of the gonadal vessels, ureter and genitofemoral nerve. At the upper limit of the left infracolic compartment, just below the attachment of the transverse mesocolon, it lies to the left of the duodenojejunal flexure. Here it curves towards the right and often raises up a ridge of peritoneum. This ridge may be excavated by a small recess of peritoneum, thus making a shallow cave, the paraduodenal recess (see p. 254).

The inferior mesenteric vein now passes behind the lower border of the body of the pancreas, in front of the left renal vein, and joins the splenic vein. Occasionally it curves to the right more sharply, and passes behind the pancreas, below and parallel with the splenic vein, in front of the superior mesenteric artery, to open directly into the superior mesenteric vein.

Lymph drainage of the gastrointestinal tract

From the whole length of the gastrointestinal tract the lymph vessels pass back along the arteries to lymph nodes that lie in front of the aorta at the origins of the gut arteries (Fig. 5.17). These comprise the coeliac, superior mesenteric and inferior mesenteric groups of lymph nodes. They drain into each other from below upwards, the coeliac group itself draining by two or three lymph channels into the cisterna chyli.

These preaortic lymph nodes are the last in a series of lymph node filters that lie between the mucous membrane of the gut and the cisterna chyli. The first filtering mechanism consists of isolated lymphoid follicles which lie in the mucous membrane of the alimentary canal from mouth to anus. They are not numerous in the oesophagus, but are numerous in the stomach, and become increasingly so along the small intestine (the MALT, see p. 9). In the lower reaches of the ileum they become aggregated together into Peyer's patches visible through the muscular wall. These lie on the antimesenteric border of the ileum and are oval in shape, with their long axes lying longitudinally along the ileum. In the large intestine the lymphoid follicles in the mucous membrane are numerous, but isolated from each other. In the appendix they are aggregated as in a tonsil.

Lymph vessels pass from the follicles in the mucous membrane through the muscle wall of the gut to nearby nodes, which also receive lymph from minute nodules on the serosal surface of the gut. The various groups of nodes are considered with the individual organs and have particular significance for the stomach and large intestine in view of the prevalence of carcinoma at these sites. However, it is convenient to note here that the small and large intestines have a common pattern of three groups of nodes. The first group lies in the peritoneum adjacent to the margin of the gut, the mural nodes in the mesentery of the small intestine, and the paracolic nodes of the large intestine. The second group of intermediate nodes lies along the main blood vessels of supply, and the third are the preaortic nodes at the origins of the coeliac and the superior and inferior mesenteric arteries. The large intestine has some additional nodes which lie on the external surface of the gut wall (and occasionally within appendices epiploicae); these are the epicolic nodes.

Nerve supply of the gastrointestinal tract

All parts of the gut and its derivatives are innervated by parasympathetic and sympathetic nerves, which travel together along the gut arteries to reach their destination. Most come from the coeliac plexus and connected plexuses on the abdominal aorta (see p. 281), but parasympathetic fibres to the hindgut come from the inferior hypogastric plexus in the pelvis (see p. 311).

From the middle third of the oesophagus to the rectum, nerve cells and fibres that supply muscle, blood vessels and glands in the alimentary tract are concentrated in two plexuses. The myenteric plexus (of Auerbach) is situated between the two muscle layers of the gut and the submucous plexus (of Meissner) is in the submucosa. Collectively the plexuses form the enteric nervous system. The system receives postganglionic sympathetic (inhibitory) and preganglionic parasympathetic (excitatory) fibres, but is unique in being able to function without these extrinsic efferent supplies, which do not pass directly to the gut muscle; the enteric system always intervenes. Afferent fibres connect with the spinal cord (by the sympathetic trunks) and with the brainstem (by the vagus). Pain impulses are transmitted by sympathetic and parasympathetic fibres, while impulses mediating sensations of distension pass in parasympathetic fibres.